<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body, ul{
      margin: 0;
      padding: 0;
    }
    body{
      width: 100vw;
      height: 56.25vw;
      box-sizing: border-box;
      border: 3px solid red;
    }

    ul{
      width: 100%;
      height: 100%;
      list-style: none;

      display: flex;
      flex-direction: row;
      flex-wrap: wrap;
    }

    li{
      width: 33.333%;
      height: 50%;
      box-sizing: border-box;
      border: 2px solid green;
      font-size: 1.5625vw;
    }
  </style>
</head>
<body>

  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
  </ul>
</body>
</html>